The present invention relates to a surface-emitting laser diode, a surface-emitting laser diode array, an electrophotographic system, a surface-emitting laser diode module, an optical communication system, an optical interconnection system using the surface-emitting laser diode, and a method of fabricating the surface-emitting laser diode.
A surface-emitting laser (a surface-emitting laser diode) has an active layer that is smaller in volume than that of an edge-emitting laser, and can perform high-speed modulation. For this reason, surface-emitting lasers are attracting more attention as light sources for communication such as Gigabit Ethernet (a trademark name). As a surface-emitting laser can have laser outputs in the direction perpendicular to the substrate, it is easy to form a two-dimensional array with surface-emitting lasers. Also, as such a two-dimensional array consumes less electricity, surface-emitting lasers are also expected to serve as light sources for parallel optical interconnection.
Conventionally, to achieve low threshold current in a surface-emitting laser, a current restricting structure has been employed. This current restricting structure is formed by an oxide of an Al mixed crystal in the following manner. For example, Non-Patent Document 1 discloses a current restricting structure formed by a selective oxidation structure in a device that includes an InGaAs/GaAs quantum well active layer formed through crystal growth by a metal organic chemical vapor deposition (MOCVD) technique, and AlAs/GaAs distributed Bragg reflectors that sandwich the quantum well active layer.
In accordance with this conventional technique, after the crystal growth of the device, etching is cylindrically performed on an area of 20 μm in diameter on the surfaces of the device and the substrate. Annealing is then performed, at 400° C., in a steam atmosphere that is formed by frothing water, heated at 80° C., with nitrogen gas. By doing so, only the AlAs layers that extend from the side of the cylindrical portion to the center of the mesa are selectively oxidized, so that an AlxOy current restricting structure is formed in the region surrounding the cylindrical portion, with a current passage having a diameter of 5 μm being maintained at the center of the cylindrical portion. As the AlxOy current restricting structure exhibits high insulation, current can be efficiently restricted to the region that has not been oxidized. With this structure, the device realizes an extremely low threshold value of 70 μA.
Further, the refractive index of AlxOy is approximately 1.6, which is lower than the refractive index of the other semiconductor layers. Because of this, a transverse-direction refraction difference is caused in the resonator structure, and oscillating light is confined in the center region of the mesa. Accordingly, diffraction loss is reduced, and the device efficiency is increased. However, the amount of confined light increases at the same time. To suppress high-order transverse-mode oscillation, the size of the oxidation confinement structure needs to be small.
In a structure having an oxidation confinement structure formed through selective oxidation performed on the low refraction layer of the distributed Bragg reflectors, the amount of confined light becomes extremely large, as the oxidation confinement structure occupies most of the area of the distributed Bragg reflectors. It is therefore necessary to form a very small oxidation confinement structure, so as to suppress high-order transverse-mode oscillation. However, a very small oxidation confinement structure causes various problems, such as an increase of resistance, as will be described later in detail. For this reason, the low refraction layers of the distributed Bragg reflectors are not oxidized in a conventional surface-emitting laser diode device that employs an oxidation confinement structure. Instead, a single selectively oxidized layer for oxidation confinement is provided in a p-type distributed Bragg reflector, in addition to the low refraction layers of the distributed Bragg reflectors, so that the amount of light confined in the transverse direction is reduced, and that the oxidation confinement structure for suppressing high-order transverse-mode oscillation has a size large enough for practical use. This structure for confining current and confining light in is commonly employed in conventional devices.
A conventional surface-emitting laser diode device that employs a single oxidation confinement structure can achieve single fundamental mode oscillation, with the diameter of the oxidation confinement structure being limited to three to four times as long as the oscillation wavelength, which might vary with wavelength band that is being used though. In this manner, decreases of the oscillation threshold and diffraction loss and single fundamental mode control are realized with an oxidation confinement structure.
As a conventional technique of performing single fundamental transverse-mode control with an oxidation structure with higher efficiency, Non-Patent Document 2 discloses transverse-mode control using an MOX (multiple oxide) structure. In accordance with this conventional technique, a p-type distributed Bragg reflector includes an oxidation structure for confining current (confining holes) and an MOX structure that is formed on the oxidation structure and has two or more oxidation structures having larger oxidation sizes. Here, the two or more oxidation structures having larger oxidation sizes are employed mainly to reduce the parasitic capacitance of the device, but also have an effect of suppressing high-order transverse-mode oscillation.
While the fundamental transverse mode exhibits great electric field amplitude at the center of the mesa, the high-order transverse mode normally exhibits great electric field amplitude in a peripheral region at a distance from the center of the mesa. If a low refraction structure is formed through oxidation in the region surrounding the mesa in this structure, high-order transverse-mode diffraction (leakage) loss is caused, and, as a result, oscillation is suppressed. Since the fundamental transverse mode exhibits great electric field amplitude at the center of the mesa that is not oxidized, the diffraction loss due to an oxidation structure having a large size is small with the fundamental transverse mode. Accordingly, the oscillation mode can be more efficiently switched to the fundamental transverse mode. In this example, a three-layer oxidation structure is employed, separately from a current restricting structure, to perform single fundamental transverse-mode control with high efficiency.
[Non-Patent Document 1]
                Electronics Letters 31 (1995), pp. 560–562[Non-Patent Document 2]        
Collection of Preliminary Lecture Manuscripts for the 47th Associated Lecture Meeting on Applied Physics, p. 29, N-2.
As described above, to achieve single transverse-mode oscillation with a current restricting structure, the size of the oxidation structure needs to be small, and the loss with the high-order mode needs to be increased. By reducing the size of the oxidation confinement structure, the threshold current can be lowered, but the area that contributes to oscillation is reduced. As a result, it becomes difficult to achieve high outputs.
In addition to the above problem, the device resistance increases with the decrease of the area of the conductive region, and high-output oscillation becomes difficult due to output saturation caused by device heat generation. Especially with a p-type semiconductor material, the effective mass of holes is great, and the mobility is low. Even if a buffer layer such as a heterospike buffer layer is inserted in the interface, the resistance of the distributed Bragg reflector is high. Because of these factors, the device resistance greatly increases with a decrease of the size of the oxidation confinement structure.
On the other hand, if a large oxidation confinement structure is formed, the oscillation region becomes broader, and accordingly, relatively high outputs can be obtained. However, such a large oxidation confinement structure cannot effectively suppress high-order transverse-mode oscillation, resulting in frequent occurrence of high-order transverse-mode oscillation.
For the above reasons, single fundamental transverse-mode oscillation cannot be achieved with high outputs. Conventionally, to achieve single fundamental transverse-mode oscillation with a single oxidation confinement structure, each side or the diameter of the oxidation confinement structure needs to be three to four times as long as the oscillation wavelength, which might vary with the wavelength band that is being used. Even with the oxidation confinement structure of such a size, the highest possible output that can be expected is only 2 mW or so.
In many cases where a surface-emitting laser is employed as a light source or a WRITE light source, such as in an electrophotographic system, an optical disk write system, and a long-distance communication using optical fibers, it is strongly desired to obtain single-peaked beams or single fundamental transverse-mode oscillation. In view of this, single fundamental transverse-mode control with a selective oxidation structure having a very small non-oxide region is essential for a surface-emitting laser diode.
As described so far, a conventional surface-emitting laser diode of an oxidation confining type simultaneously performs current restricting and single fundamental transverse-mode control with one selective oxidation structure having a very small non-oxide region. Although such a surface-emitting laser diode can achieve very low threshold current and signal fundamental transverse-mode oscillation, the device resistance is very high. Furthermore, it is difficult to have high outputs, because of the smaller oscillation region and an increase of heat generation due to the higher resistance. As the device resistance is very high, it is also difficult to perform high-speed modulation.
In the structure disclosed in the Non-Patent Document 1, which employs an oxidation confinement structure formed through selective oxidation performed on the low refraction layers of the distributed Bragg reflectors, the amount of confinement is too large, and the size of the oxidation confinement structure needs to be reduced to suppress high-order transverse-mode oscillation. Further, as the oxidation confinement structure that confines holes occupies most of the area of the p-type distributed Bragg reflector, the device resistance becomes very high, and it is very difficult to perform high-output operations.
With the MOX structure disclosed in Non-Patent Document 2, on the other hand, high-order transverse-mode oscillation can be suppressed, and the parasitic capacitance can be reduced. However, since a number of oxidation confinement structures are provided in one p-type Bragg reflector, the device resistance becomes very high. Also, as an oxidation confinement structure of a small size is employed as the hole restricting structure, the device resistance is still very high, and it is difficult to have high outputs.